For the monitoring of co-ordinate measurement machines and production devices, measurements on test objects are required which acquire the main deviations of the measurement machine or the production device. As mechanical comparative standards, such test objects represent an economical alternative to measuring comparative standards, such as for example interferometers. Although measurements with interferometers supply reliable information on the precision of the inspected machines, such a method in practice is very time consuming, so that the monitoring intervals are often chosen to be very long, e.g. annual. Modern machine tools and industrial robots operate in tight tolerance ranges and sometimes exhibit drift characteristics so that more frequent checking is necessary. With mechanical test objects additional inspections at shorter time intervals are possible also under economical viewpoints.
Depending on the field of application, various requirements are placed on test objects. In particular they should provide reliable measurement results within the scope of standard measurement conditions, i.e. at a temperature of −20° C. to +70° C. and a relative humidity of 0% to 100%, in order that they can be employed under various ambient conditions. Furthermore, the dimensions to be measured are sometimes very different. With large equipment to be measured they may extend into the range of some metres to over ten metres. Another viewpoint is the flexibility and the mobility of the test object which is why a large test object should preferably be able to be disassembled for transport and should be as light as possible, wherein however the accuracy of the measurements on the test object should be ensured.
A mechanical test object is described in DE 199 15 012 A1. It consists of four probe form elements and six connecting elements which are combined in a tetrahedral shape, so that the probe form elements are located at the corners of the tetrahedron. Each connecting element is located between two probe form elements. The materials of this test object are chosen such that a linear thermal expansion coefficient from probing point to probing point arises which is essentially equal to zero. Here, due to the design, the connecting elements are equally long in order to provide a self-supporting structure through the special shape of the tetrahedron, maintaining the probe form elements in well defined positions. This type of test object can be disassembled, the probe form elements are made of steel or glass ceramics and the connecting elements are of a light material, i.e. carbon-fibre reinforced plastic (CFRP), whereby good transportability is ensured. In one embodiment the releasable connections of the probe form elements to the connecting elements are based on magnetic forces.
The parts of the tetrahedron can however also be combined in that a number of connecting elements are arranged one behind the other, with in each case a probe form element between them and one at each of the two ends of the linear arrangement, forming a ball bar. Here, up to three additional probe form elements are optionally employed. The probe form elements may have various designs depending on the field of application, e.g. in the form of a ball or a different shape. Then probing points at intervals of integer multiples of the distance between two adjacent probe form elements are available, wherein the minimum distance is determined by the length of one of the connecting elements of the same length and the maximum length is six times the minimum distance.
To incorporate a ball bar thus formed into the measurement volume a holder is needed for reasons of stability and adjustment. According to the state of the art, this holder consists in this case of single seats onto which the probe form elements are placed, wherein however an adequate linear alignment of the ball bar must be achieved with low deviations in alignment.
The probing of the probe form elements occurs through tactile contact, i.e. they are for example probed with measuring styli through direct contact. However, amongst the state of the art are also elements measured by light. Generally, here a measurable element of a test object is designated a target. For measurements with tactile systems, chromium or stainless steel balls are, for example, used and the determination of the centre point of the ball occurs via a ball measurement. Also so-called reset targets can be measured by tactile systems, wherein the centre point of the ball is obtained using a cone in which a small ball with a defined diameter is placed so that the centre point of the target can be directly probed via a simple point measurement. If the probing is carried out using light, so-called retro-targets or theodolite targets are employed, for example, for measurements with photogrammetric and other optical systems. Finally, prisms can also be used for measurements with a laser tracker as target.
Other ball bars are also known from the state of the art, wherein the probe form elements are balls consisting of ceramic held on a carrier body at uniform distances by leaf-spring elements firmly joined to the carrier body. The exact distance between the balls is provided by distance tubes of steel which are clamped between the balls.
From the state of the art a holder for linear ball bars with fixed ball mountings at constant ball distances and the application of independent single seats is known.
Other known test objects, which cover two or three spatial dimensions, are designed in the form of a ball plate or ball cuboid, wherein the distances between the balls are permanently specified.
The test objects known from the state of the art have either the disadvantage of being inflexible due to the given ball distances or of being unusable due to a holder that is unsuitable for many purposes.
With the holder with fixed ball mountings it is disadvantageous that the distances of the balls are defined and constant due to the fixed position of the ball holders on the carrier body. Adaptation of the ball distances to the relevant measurement requirements is therefore not possible.
If the balls and bars are clamped, then stresses arise which can impair the measurement result, which is a disadvantage. Furthermore, a poorer temperature neutrality when using steel as the material for the distance tubes is also disadvantageous.
If a ball bar is placed on single seats, then adequate linear alignment of the ball bar must be ensured in order to achieve a low alignment error, which can only be obtained conditionally and with difficulty using seats which have to be adjusted independently of one another and which therefore is disadvantageous.
From the state of the art, as described above, only test objects are known which exhibit fixed distances between the targets. Here, the distances are already defined by the construction. In the case of ball bars with variable lengths of the connecting elements, such a construction is either unstable and therefore is not easily used or difficult to align and can only be positioned as required in the measurement volume with a great deal of adjustment.
Variable test objects, which cover one or more dimensions, and an associated stable holder for such test objects which can be arranged flexibly are not known from the state of the art. From the state of the art only one holder for linear ball bars with fixed ball mountings at constant ball distances or the use of independent single seats is known.